Page 36 - Occupational Health & Safety, June 2017

ELECTRICAL SAFETY
idea, there have not been specific requirements for it in industry codes and standards until recently. However, NEC 240.87 was sig- nificantly revised in 2014 and now requires Arc Energy Reduction means to be applied on LV circuit breakers 1200A or larger. In the next section of this article, we will explore options for mitigating arc flash hazards that may be used to meet this requirement or for other mitigation purposes.
The Role of the Circuit Breaker or Fuse
in Lowering Arc Flash Energy Levels
Why is a circuit breaker or fuse always considered in the arc flash analysis? Because arcing time is the key determining factor for arc flash energy. Per the equations in IEEE Std. 1584-2002, arc flash in- cident energy varies linearly with time. If the duration of the arcing fault doubles, the available energy doubles; halve the duration and you cut the energy in half.
Because incident energy is proportional to arcing time, the use of faster-acting devices is key. As a result, proper selection of overcurrent protective devices—in particular, selecting devices that will quickly clear arcing faults from the power system—is a powerful mitigation strategy.
Over-Current Protective Device (OCPD) Coordination Study
An OCPD coordination study optimizes the protective device set- ting for reliability and arc flash protection. While an OCPD study is not a requirement of an arc flash analysis, it is recommended to
have this study completed as a component of an arc flash analysis. The OCPD coordination study will determine whether minor ad- justments in circuit breaker (or other over-current protective device) settings can lower incident energy levels. However, settings must be chosen to properly protect equipment while still allowing for normal load currents and routine temporary overcurrents (e.g., motor start- ing current) to flow without causing a trip.
Specialized Relaying, Such as Optical Technology
Quickly clearing faults is a key to arc flash mitigation. Circuit breaker or relay settings near the source of power may have significant time delays to allow for coordination of downstream devices. A relatively new way to address this issue is to use relays that detect the presence of arcing faults by looking for the flash of light associated with the arcing fault in addition to the characteristic current flow.
However, for an arcing fault to be detected, both the high current and a burst of light must exist. When both conditions are present, the optical relay can operate very quickly and send a signal to an upstream breaker or interrupter to clear the fault. Optical relays also can be used as the protective relay in a virtual main configuration.
Virtual Main Arc Flash Mitigation Systems
Switchgear and switchboards can be subjected to dangerous levels of arc flash incident energy when fed directly from a power trans- former. The addition of a virtual main system, where a digital re- lay and overcurrent sensing is added to the low-voltage side of the
One Mission.
One Focus.
No Distractions.
Emergency equipment isn’t just what we do. It’s all we do. Your safety is too important to entrust to just anyone. All Guardian emergency eyewash and shower products are inspected, fully assembled, and 100% water-tested in our Chicago facility, which means you receive the most reliable emergency equipment each time, every time.
Your safety isn’t just a priority. It’s our only priority.
Visit gesafety.com to learn more.
Circle 36 on card. See us at ASSE, Booth 739
32 Untitled-2Oc1cupational Health & Safety | J U N E 2 0 1 7 w w w.5o/h3/s17o n1l i1n:1e3. AcMo m